WO2004067809A1 - Method for regeneration of an electrolysis bath for the production of a compound i-iii-vi2 in thin layers - Google Patents

Abstract

The invention relates to the regeneration of an electrolysis bath for the production of I-III-VIY compounds in thin layers, where y is approaching 2 and VI is an element including selenium, whereby selenium is regenerated in the form Se(IV) and/or with addition of oxygenated water to reoxidise the selenium in the bath to give the form Se(IV).

Description

 Method for regenerating an electrolysis bath for the manufacture of a compound I-III-VI2 in thin layers

The present invention relates to the manufacture of semiconductors of type I-III-VI ₂ in thin layers, in particular for the design of solar cells.

Compounds I-III-VI ₂ of the CuIn _x Ga (i- _X ) Se _y S ( _{2. Y} ) type (where x is substantially between 0 and 1 and y is substantially between 0 and 2) are considered to be very promising and could be the next generation of thin-film photovoltaic cells. These compounds have a direct prohibited bandwidth of between 1.05 and 1.6 eV which allows strong absorption of solar radiation in the visible.

Record yields of photovoltaic conversion were obtained by preparing thin layers, by evaporation on small surfaces. However, evaporation is difficult to adapt to the industrial scale due to problems of non-uniformity and low use of raw materials. Sputtering (so-called "sputtering" method) is better suited to large areas, but it requires vacuum equipment and very expensive precursor targets.

There is therefore a real need for alternative techniques at low cost and at atmospheric pressure. The technique of depositing thin layers by electrochemistry, in particular by electrolysis, is presented as a very attractive alternative. The advantages of this deposition technique are numerous and in particular the following:

- deposit at ambient temperature and pressure in an electrolysis bath,

- possibility of treating large areas with good uniformity,

- ease of implementation,

- low cost of installation and raw materials (no particular shaping, high rate of use of materials), and

- wide variety of possible forms of deposit, due to the localized nature of the deposit on the substrate.

Despite numerous researches in this direction, the difficulties encountered have concerned the quality control of the electrodeposited precursors (composition and morphology) and the efficiency of the electrolysis bath after several successive deposits.

An object of the present invention is to provide a process for manufacturing thin layers of a compound 1-III-VI.y (where there is close to 2) by electrolysis, which ensures the stabilization and the reproducibility of the deposition conditions.

An underlying aim is to be able to carry out on large surfaces a large number of successive deposits of thin layers having the desired morphology and composition. Another object of the present invention is to provide a process for manufacturing thin layers of the compound I-III-VI _y , which ensures a satisfactory lifetime of the electrolysis bath, as well as an efficient regeneration of the raw materials consumed during electrolysis.

Another object of the present invention is to provide a process for manufacturing thin layers of the compound I-III-VI _y , which ensures regeneration of the raw materials consumed during electrolysis, without however unbalancing the composition of the electrolysis bath. and then reduce its lifespan.

To this end, it proposes a process for manufacturing a compound I-III-VI _y in thin layers by electrochemistry, where y is close to 2 and VI is an element comprising selenium, of the type comprising the following steps: a) providing an electrolysis bath comprising active selenium, of oxidation state IV, as well as at least two electrodes, and b) applying a potential difference between the two electrodes to substantially promote a migration of the active selenium towards one of the electrodes and thus initiate the formation of at least one thin layer of I-III-VI _y .

The method within the meaning of the invention further comprises a step c) of regeneration of selenium in active form in said bath, in order to increase a lifetime of said electrolysis bath. Thus, within the meaning of the present invention, one begins by regenerating the active selenium bath before regenerating it into element I (such as copper) and / or element III (such as indium or gallium). Indeed, it was found that a low re-introduction of active selenium in the bath (preferably, an excess' of approximately 20% by molar concentration ^• based on the amount normally added selenium) achieved with again substantially the same number and the same volumes of thin layers as those obtained at the end of step b).

Advantageously, at the end of step c), at least one new thin layer of I-III-VI _{y is formed} .

Thus, in a first embodiment, in step c), selenium is added to the bath to form an excess of active selenium in the bath.

In another embodiment, variant or complementary to the above-mentioned first embodiment, in step c), an oxidant of selenium is introduced into the bath, in order to regenerate selenium in active form.

Usually, the electrolysis bath, when it ages during deposition, exhibits selenium colloids. This selenium in the form of colloids has an oxidation state of 0 and, in the context of the present invention, is not capable of combining with elements I and III. Advantageously, if the bath comprises selenium in the form of colloids in step b), the abovementioned oxidant is capable of regenerating selenium in the form of colloids, into selenium in active form.

Thus, it will be understood that the term “selenium in active form” is understood to mean selenium at the oxidation state IV, capable of being reduced to the electrode in ionic form Se ^{2 "} and of combining naturally with the elements I and III to form the thin layers of I-III-VI _y , and being distinguished from selenium of oxidation state 0, for example in the form of colloids in the bath solution, which does not combine with elements I and III.

In a particularly advantageous embodiment, said oxidant is hydrogen peroxide, preferably in concentration in the bath of an order of magnitude corresponding substantially to at least five times the initial concentration of selenium in the bath.

The addition of hydrogen peroxide in the bath then makes it possible to regenerate the electrolysis bath at very low cost. In addition, this regeneration is carried out without pollution of the bath since a simple degassing makes it possible to find the initial constitution of the bath.

With this in mind, where the electrolysis bath is regenerated by limiting its pollution by the regenerating additives, a step subsequent to step c) is advantageously provided for regenerating the electrolysis bath by the introduction of oxides and / or of hydroxides of elements I and III. Other advantages and characteristics of the invention will become apparent on reading the detailed description below of embodiments given by way of nonlimiting examples, as well as on examining the accompanying drawings and in which :

- Figure 1 shows schematically a thin layer obtained by the implementation of the method according to the invention, and - Figure 2 shows schematically an electrolysis bath for the implementation of the method according to the invention.

With reference to FIG. 1, CO layers of copper and indium diselenide are obtained at ambient pressure and temperature by electrodeposition of a thin layer of precursors of suitable composition and morphology, on a glass substrate S covered with molybdenum MO. The term "precursor layer" is understood to mean a thin layer of overall composition close to CuInSe ₂ and directly obtained after deposition by electrolysis, without any subsequent treatment.

The electrodeposition is carried out from an acid bath B (FIG. 2), stirred by blades M, containing an indium salt, a copper salt and dissolved selenium oxide. The concentrations of these precursor elements are between 10 ^"4 and 10 ^{" 2} M. The pH of the solution is fixed between 1 and 4.

Three electrodes An, Ca and REF, of which a Ca molybdenum electrode (for cathode) on which the thin layer is formed by electrodeposition, - and a reference electrode with mercury sulphate REF, are immersed in bath B.

The difference in electrical potential applied to the molybdenum electrode is between -0.8 and -1.2 V compared to the reference electrode REF.

Layers of thickness between 1 and 4 microns are obtained, with current densities between 0.5 and 10 mA / cm ² .

Under defined conditions of composition, agitation and potential difference, it is possible to obtain dense, adherent layers, of homogeneous morphology and whose composition is close to the stoichiometric composition: Cu (25%), In ( 25 + ε%) and Se (50%), with a composition slightly richer in indium, as shown in Table I below. We can thus make deposits on surfaces of 10x10 cm ² .

An exemplary embodiment of

1 invention.

A typical deposit is made from a bath, the initial formulation of which is as follows:

[CuSO ₄ ] = 1.10 ^~ 3M,

[In ₂ (SO _{4) 3]} = 3,0.10- ³ M, [H ₂ SeO _3] = l, 7.10 ^"3 M,

[Na ₂ SO ₄ ] = 0.1 M, where the notation "M" corresponds to the unit "mole per liter", for a pH of 2.2.

The precursors are deposited by a cathodic reaction with an imposed potential, at −1 V relative to the REF electrode. The current density is -1 mA / cm ² .

After each electrolysis, the bath is recharged with Cu, In and Se elements based on the number of coulombs indicated by a detection cell (not shown) which thus counts the number of ions having interacted in the bath solution. This recharging keeps the concentration of the elements constant during successive electrodeposits. The pH can also be readjusted by adding sodium hydroxide (such as NaOH, for a concentration such as 1M) but this measurement is not systematically necessary here, as will be seen below.

Under these conditions, it is usually found that after an indication of 500 + 100 Coulombs in a solution of one liter (corresponding to the electrodeposition of 4 to 5 thin layers of 25 cm ² having a thickness of 2 μm), detachment partial or total layers of CuInSe ₂ appear systematically.

According to the invention, this separation disappears by regenerating the selenium bath, even before regenerating the Cu and In elements. A distinction should be made here between active selenium with degree of oxidation IV, usually denoted Se (IV), and inactive selenium with degree of oxidation 0, which is generally observed in the form of colloids in the electrolysis bath. and usually noted Se (0).

It is indicated that the active selenium Se (IV) is the only one capable of being reduced to the Ca electrode in the ionic form Se ^{2 "} and of combining, in this form, with the elements Cu and In to form the thin layers of CuInSe ₂ .

It is also indicated that there are two competitive reactions during the electrolysis: the selenium introduced into the bath can transform at the electrode:

- either in Se ^{2 "} favorable to the formation of thin layers as indicated above,

- either in Se (0) in the form of colloids, which is unfavorable for the formation of thin layers, in particular because colloids pose problems at the interface between the substrate (or the MO layer of molybdenum here) and the layer thin Cu-In-Se in formation.

Advantageously, we. regenerates excess Se (IV) in the bath. To this end, selenium oxide dissolved in the electrolysis bath is added to delay aging of the bath. In practice, for a thin layer formed and 115 Coulombs passed into the solution, it is theoretically necessary to add 1.8.10 ^"4 M of [H ₂ Se0 ₃ ] to the solution to find an initial selenium concentration of 1.7.10 ^{" 3} M An addition of double this quantity (ie 3.6.10 ^"4 M and therefore an excess of 1.8.10 ^{" 4} M of [H ₂ Se0 ₃ ]), on the fifth deposit, makes it possible to obtain adherent layers again. These thin layers have the composition (Table I) and the desired morphology. An overgeneration of 3.6.10 ^"4 M thus makes it possible to obtain a cycle of 4 to 5 layers of satisfactory adhesion before observing new problems of delamination. After each delamination cycle, the renewal of this operation makes it possible to obtain adherent layers.

As a variant or in addition to this operation, an oxidant is used which allows the selenium to be re-oxidized in the form Se (0), in order to obtain selenium in the form Se (IV). For this purpose, use is preferably made of hydrogen peroxide H ₂ 0 ₂ , by putting a large excess of H ₂ 0 ₂ in the solution (concentration of the order of 10 ^"2 M, preferably close to 4.10 ^{" 2} M). The layers become adherent again for 4 to 5 successive deposits of thin layers, then peel off again. The renewal of this operation also makes it possible to again obtain adherent layers. Advantageously, it has been observed that the addition of hydrogen peroxide also makes it possible to obtain thin layers of relatively smoother morphology.

We thus note a great similarity of the effects which an over-regeneration in Se (IV) and the addition of H ₂ 0 ₂ bring in the solution. It is further indicated that other types of oxidant than hydrogen peroxide, in particular ozone 0 ₃ , can be used to increase the life of the baths. The composition (Table I) and the morphology of the layers is substantially the same, whether hydrogen peroxide has been added to the bath or that a regeneration of selenium (IV) has been carried out.

Table I Comparative analysis of the composition of the thin layers of CuInSe ₂ electrodeposited as a function of an excess over-regeneration of selenium Se (IV) and of an addition of hydrogen peroxide.

The addition of hydrogen peroxide or the regeneration in excess of Se (IV) make it possible to considerably increase the number of layers that can be deposited with a bath. Such recycling of the bath makes it possible to consume all of the elements introduced, and more particularly indium, by electrolysis, which makes it possible to reduce in a particularly advantageous manner the costs of manufacturing the precursors, in particular compared to the methods of evaporation or sputtering.

It is indicated that, according to an advantageous aspect of the regeneration of the bath within the meaning of the invention, oxides or hydroxides of copper and / or indium are also added for regenerate the CuInSe ₂ electrolysis bath into copper and / or indiu.

For example, by adding the oxides of copper CuO and indium ln ₂ 0 ₃ to the bath, the following reactions (1) and (2) are formed:

CuO + H ₂ 0 -> Cu ²⁺ + 20H ^" (1) •

(l / 2) In ₂ 0 ₃ + (3/2) H ₂ 0 → In ³⁺ + 30H ^" (2)

On the other hand, if the CuS0 ₄ and In ₂ (S0 ₄ ) ₃ compounds were added, the bath would be polluted with sulfate ions S0 ^{2 ~} .

In addition, the reaction for forming CuInSe ₂ at the cathode is written: Cu ²⁺ + In ³⁺ + 2H ₂ Se0 ₃ + 8H ⁺ + 13e ^" -> CuInSe ₂ + 6H ₂ 0 (3) where e ^" corresponds to the notation of an electron, while at the anode, we have the following reaction:

(13/2) H ₂ 0 → 13H ⁺ + (13/4) 0 ₂ + 13e ^" (4) to respect the load balance.

It is then noted, according to another advantage that the addition of oxides of Cu and In provides, that the difference of five excess H ⁺ ions according to equations (3) and (4) is compensated by the five OH ions ^" introduced by reactions (1) and (2). It will thus be understood that the addition of Cu and In oxides also makes it possible to stabilize the pH of the solution and to dispense with the addition of sodium hydroxide as indicated below. -before. It is further indicated that the addition of Cu (OH) ₂ and In (OH) ₃ hydroxides produces the same effects, reactions (1) and (2) simply becoming:

Cu (OH) ₂ → Cu ²⁺ + 20H ^" (1 ') In (OH) ₃ → In ³⁺ + 30H ^" (2')

This ensures durability and stability of the electrodeposition baths of compounds I-III-VI _y such as Cu-In-Se _y (with y close to 2) by adding agents which do not affect the quality of the layers . The layer of electrodeposited precursors contains the elements in composition close to the stoichiometry I-III-VI ₂ . The compositions and morphology are checked during electrolysis. These agents (excess of Se (IV) or H ₂ 0 ₂ ) can easily be used for any type of electrolysis bath allowing the electrodeposition of I-III-VI systems such as Cu-In-Ga-Al-Se- S.

The conversion yields obtained (9% without an anti-reflection surface layer) attest to the quality of the deposits obtained by the process according to the invention.

Of course, the present invention is not limited to the embodiment described above by way of example; it extends to other variants.

Thus, it will be understood that the elements I and III initially introduced into the solution in the form CuS0 ₄ and In ₂ (S0 ₄ ) 3 can advantageously be introduced rather in the form of oxides or hydroxides of copper and indium for limit the pollution of the bath.

Claims

1. Method for manufacturing a compound I-III-VI _y in thin layers by electrochemistry, where y is close to 2 and VI is an element comprising selenium, of the type comprising the following steps: a) providing a bath of electrolysis comprising active selenium, of oxidation state IV, as well as at least two electrodes, and b) applying a potential difference between the two electrodes to appreciably promote a migration of the active selenium towards one of the electrodes and thus initiate the formation of at least one thin layer of I-III-VI _y , characterized in that it further comprises a step c) of regeneration of selenium in active form in said bath, to increase a lifetime of said bath electrolysis.

2. Method according to claim 1, characterized in that, in step c), an oxidant of selenium (Se (0)) is introduced into the bath, in order to regenerate selenium in active form (Se (IV)).

3. Method according to claim 2, characterized in that, the bath comprising selenium (Se (0)) in the form of colloids in step b), said oxidant is arranged to regenerate selenium (Se (0)) under form of colloids, selenium (Se (IV)) in active form.

4. Method according to one of claims 2 and 3, characterized in that said oxidant is hydrogen peroxide (H ₂ 0 ₂ ).

5. Method according to claim 4, characterized in that the concentration of hydrogen peroxide added to the bath is of an order of magnitude corresponding substantially to at least five times the initial concentration of selenium in the bath.

6. Method according to one of claims 1 to 5, characterized in that, in step c), selenium is added to the bath to form an excess of active selenium in the bath.

7. Method according to claim 6, characterized in that, for substantially one tenth of the concentration of selenium in step a) consumed by manufacturing at least one thin layer in step b), is added to the bath , in step c), substantially twice the concentration consumed.

8. Method according to one of the preceding claims, characterized in that, at the end of step c), at least one new thin layer of I-III-VI _{y is formed} .

9. Method according to one of the preceding claims, characterized in that, for the manufacture of thin layers of CuInSe _y , the bath comprises in step a), for a unit of concentration of copper in the bath, approximately 1, 7 active selenium concentration units.

10. Method according to one of the preceding claims, characterized in that it comprises a step subsequent to step c), of regeneration of the electrolysis bath by the introduction of oxides and / or hydroxides of elements. I (CuO; Cu (OH) ₂ ) and III (ln ₂ 0 ₃ ; In (OH) ₃ ).